The single-frequency observations of the Sun on metric wavelengths are useful for calibration of a dynamic spectrum records and for the study of Type IV bursts. It is widely known that these observations are still not satisfactorily calibrated. There is a great disagreement of the measured fluxes as reported from various stations. The general situation was reviewed in the report "Radio Monitoring of the Sun" by H. Tanaka, presented to IAU – Sydney 1973 and details regarding microwave region in the W. G. Report – Commission 5 URSI – Warsaw 1972.
The prevailing worldwide situation in meter wave solar observations and associated data reduction problems lacked improvements for a considerable span of time. It seems that the most important contributions to inaccuracy of measurings result from
a/ nonuniformity of the galactic background radiation /temporal immersion of the Sun in the Galaxy/,
b/ large dynamic range of the solar bursts,
c/ man-made interference,
d/ lack of good aerial calibration methods,
e/ broad beam-width of small antennas /ground reflections/,
f/ subjectiveness of the data reduction.
Some kinds of receiving systems enable measurements of some parameters of the solar radiation with sufficiant accuracy as a result of applying special techniques. The galactic background problem can for example be avoided by use of the interferometric techniques. Problem b/ can be dealt by automatic attenuators or AGC. However, AGC beeing nonlinear introduces great errors and requires inconvenient calibration sources. Narrowing the bandwidth in the HF section of the radiometer sometimes enables avoiding the interference problem, but it reduces sensitivity.
The absolute calibration of the flux is still not entirely satisfactory. The use of Cas A or Cyg A as reference sources for the northern hemisphere is usually connected with a change of the antenna beam in the elevation angle and this may introduce some additional errors. Furthermore, the flux of Cas A is decreasing with time and the decay rate as well as its dependence on frequency are not accurately known. Maybe Vir A and Tau A would be better standards of absolute flux for longer waves. They lay near the ecliptic and equator planes and their flux does not show any time variations and linear polarization /at least that of Vir A/. However their fluxes are about 10 times lesser than those of Cas A and Cyg A.
There were suggestions to standardize some of parameters of receiving systems to avoid nonuniformity of results reported from various stations. It seems that the LF integrating time constant and the recorders tape speed or the sampling frequency should be considered at first since they strongly affect the shape and peak flux of burst records and their time resolution.
Finally, significant improvements in data processing are highly desirable as observers are losing interest in the handling of the great amount of recordings.
Torun, April 27th, 1976. | K. M. Borkowski | |
1. Antenna system: type, mounting, gain, HPBW /e.g. 5 m paraboloid, equatorial, 12 dB over dipole, 40°/.
2. Radiometers: type, frequency, gain, HF or IF bandwidth, dynamic range /dBs over sensitivity/, noise temperature excess in deg/, LF time constant, monitoring /e.g. Dicke, 100 MHz, 130 dB, 250 kHz, 20 dB, 300 K, 3 s, acoustic/.
3. Recorders: type /paper-chart, magnetic, digital/, speed of tape or sampling, dynamic range /greatest to least detectable signals/, quantity /e.g. 2 one-channel paper-chart, 20 cm/h, 100 : 1/.
4. Methods used to cover the large dynamic of the bursts /e.g. gain reduction by AGC; n-stage attenuator switching/.
5. Methods used to combat interference /e.g. change of band/.
III. OBSERVATIONS
1. Type /total flux, polarization/.
2. Observing periods /especially in the winter time, e.g. always from sunrise to sunset/.
3. Daily calibration /e.g. every 3 hours with the use of the noise diode/.
4. Absolute calibration /against Cas A/Cyg A/Vir A/... every n months assuming for flux of ...; against artifical source assuming .../.
IV. DATA PROCESSING /manually, computing, eyeing/
1. Mean fluxes
2. Outstanding occurrences
3. Polarization
4. Variability /assumed criteria of evaluation/
5. Estimated accuracies of determinations /e.g. 25 % of each parameter/
V. MISCELLANEA: Publication of the results /monthly reports, local bulletin/, references for more detailed descriptions, name and address for further correspondence.